Date of Award
Doctor of Philosophy (PhD)
Dr. Susan A. Dudley
Roadsides are an interesting and unusual habitat. Plants inhabiting these degraded areas are exposed to many easily identifiable and unnatural selection pressures. In this thesis, I examine the evolutionary ecology of three species (Dipsacus sylvestris, Hesperis matronalis and Asclepias syriaca) of common roadside plants. I collected seeds from several maternal plants from three oldfield and three roadside populations. In all six studies, I utilized a similar experimental design with analyses based on the variation between maternal families. I first compared the salt and manganese tolerance during germination of oldfield and roadside genotypes of all three species. I found that one population of H matronalis possessed a greater ability to tolerate high manganese levels than other roadside populations or oldfield populations. In addition, all three roadside populations of D. sylvestris had a greater ability to tolerate high salinity than oldfield populations. Some maternal families actually grew longer roots under high salinity than under control conditions. Examination of seed characteristics revealed a significant correlation among roadside maternal families in the amount of sodium leached from the seeds and the degree of salt tolerance or salt enhanced growth indicating that the salt tolerance could be a maternal effect rather than a genetic trait. I then attempted to discern the role of seed composition, which is controlled primarily by the maternal plant, in conferring salt tolerance to roadside populations of D. sylvestris. I leached the soluble components of the seed prior to salt exposure and then compared the ability of oldfield and roadside families to tolerate high salinity. I found that there were several salt tolerance mechanisms present among the roadside populations and that the maternal environment was likely involved in conferring the salt tolerance in some instances. I established a reciprocal transplant experiment between the roadside and oldfield involving all three species. I did not observe any evidence that adaptation to roadside conditions had occurred in roadside populations of any of the three species. However, the experiment was established in the late spring after the peak salinity levels. I speculated that adaptations in D. sylvestris to the high salinity of roadside soils may be visible if the seeds spent the winter and early spring in the field. I then established a second reciprocal transplant experiment involving only D. sylvestris between the oldfield and the roadside but planted the seeds in late fall to early winter. Again, I observed no evidence of local adaptation to the roadside environment. However, seed size, a trait controlled by the maternal plant, appeared to be very important in determining success during seedling establishment in both oldfield and roadside locations and among both oldfield and roadside genotypes. I then attempted to determine the role of tolerance to low water potentials in conferring salt tolerance in roadside populations of D. sylvestris. I found that there was no relationship between the ability to tolerate low water potentials and the ability to tolerate high salinity. An additional, unintended treatment of hypoxia inflicted by one of the osmotic stress treatments, also revealed that roadside populations of D. sylvestris possess considerable variation for the ability to tolerate hypoxia stress. Again, there was no relationship between hypoxia tolerance traits and salt tolerance or tolerance of low water potentials. I then questioned whether the high sodium levels present in the seeds of roadside D. sylvestris maternal families could be detrimental to germinating seeds. I compared the toxicity of the seed leachate of oldfield and roadside maternal families on other germinating D. sylvestris seeds from both the oldfield and the roadside. I found that the leachate of roadside D. sylvestris seeds inhibited the root growth of oldfield seeds but, the leachate of oldfield seeds had no impact on the root growth of roadside seeds. These studies strongly indicated that high salinity is a strong selection pressure in roadside communities. However, many stresses are impacting the vegetative community of roadside simultaneously. The selection pressures created by the stresses may be pushing populations in multiple conflicting directions. Different populations and even different families within populations are responding to different selection pressures. In addition, the lack of overlap in the tolerance traits suggests that adaptation to one selection pressure may come at the cost of adaptation to a different pressure. The action of the many different selection pressures may prevent local adaptation from being observed in field studies. In conclusion, it is very clear from these studies that roadside plant populations are evolving to better tolerate the unusual environmental conditions created by roads and a unique vegetative community is developing in this habitat.
BEATON, LAURA LOUISE, "EVOLUTIONARY CHANGE IN THREE SPECIES OF COMMON ROADSIDE PLANTS" (2004). Open Access Dissertations and Theses. Paper 1616.